In a relaxation oscillator, the waveform is generated by charging and discharging a capacitor through which elements?

• A. A resonant LC tank
• B. A transformer and diode network
• C. A comparator or Schmitt trigger
• D. A voltage-controlled oscillator with an LC tank

Answer: (C)

A relaxation oscillator relies on a threshold-switching element that lets the capacitor charge and discharge in a controlled, repeating cycle. A comparator or Schmitt trigger fits this role perfectly: as the capacitor voltage climbs and crosses the upper threshold, the comparator output flips, reconfiguring the circuit so the capacitor discharges rapidly toward the lower threshold. When that lower threshold is reached, the output flips again, and the cycle repeats. This creates the characteristic waveform with slow voltage ramps set by the RC time constant and fast transitions driven by the switching action.

Other options don’t provide that clean, abrupt threshold-triggered switching. An LC tank or a transformer/diode network relies on energy exchange or nonlinearities rather than a defined, hysteresis-based trigger to create the charging/discharging cycle, and a voltage-controlled oscillator with an LC tank is itself a resonant oscillator rather than a relaxation oscillator.